This invention relates in general to mobile radio communications systems utilizing repeaters. More specifically, this invention relates to mobile communications systems wherein mobile stations communicate with one another and with a central dispatcher through a plurality of repeater stations.
When radio communication coverage over large geographic regions or over rough topography is required, it is frequently necessary to utilize a plurality of remote radio repeater stations, each including a receiver and some of which include a transmitter. One such system is illustrated by U.S. Pat. No. 2,734,131, granted Feb. 7, 1956, incorporated herein by reference. That patent discloses a communication system with carrier strength control. A mobile user is able to communicate with a central station through any one of a plurality of repeater stations. A relay arrangement forces two-way communication between the dispatcher and mobile user to be established through the repeater station receiving the strongest signal from the mobile user.
In multiple remote repeater station communications systems, the transmitters within the remote repeater stations may re-transmit received signals or they may transmit dispatcher-originated signals intended for reception by one or more mobile users. Various arrangements are now utilized for enabling and disabling the repeat function and for controlling the various modes of operation of these remote repeater stations (also known as satellite stations) in response to signals received thereby.
Radio signals generated by a mobile station may be received by several repeater stations simultaneously, but one repeater station may receive a mobile-originated signal with a greater signal strength and clarity than another repeater station. Therefore, control arrangements for repeater systems generally include a voter for selecting the strongest received signal detected by a repeater station for coupling via a telephone line to the dispatcher's central station. Thus, the dispatcher is provided the best opportunity to receive and understand a message originated by a mobile user. Presently utilized control systems generally include a central control console such as the MASTR Controller Series 539 and 549 provided by the General Electric Company (MASTR is a registered trademark of the General Electric Company). Details of the General Electric MSTR Controller Series 539 and 549 are set forth in maintenance manual LBI 4478K, data file folder DF 4094, available from the General Electric Company, Mobile Communications Business Division, Lynchburg, Va. The contents of that manual are incorporated herein by reference. A typical control arrangement is shown in FIG. 1.
Referring now to FIG. 1, there is shown a typical known arrangement for controlling two remote repeater stations 10 (10-1 and 10-2). Each of remote repeater stations 10 include a transmitter 12 (12-1 and 12-2, respectively) and a receiver 14 (14-1 and 14-2, respectively). Each repeater 10 has an antenna 16 (16-1 and 16-2, respectively) serving both transmitter 12 and receiver 14. Receiver 14 of each repeater stations 10 is capable of detecting signals originated by a mobile station and providing the detected signals at an output 17 (17-1 and 17-2, respectively) thereof. Mobile communications systems of this type are generally used by field service organizations, police and fire organizations, taxicab organizations, and the like. Therefore the signals are general speech, i.e., audio that is amplitude or angle modulating an RF carrier. However, such systems could be utilized for other forms of communication, such as coded digital data streams, and the like. Therefore, it is to be understood that, as used throughout, the term "signal" is intended to be read in its broadest sense. Signals originated by a mobile station are detected via antennae 16 and receivers 14 and a demodulated audio signal is provided at output 17 of the receivers.
The audio signal from each of receivers 14 is coupled to a voter 18 via a separate and distinct telephone line 20 (20-1 and 20-2, respectively). Voter 18, common to all remote repeater stations 10 is located at some central location serving all remote repeater stations 10. This central location may be in the same general area as central control console 22.
Voter 18 compares the received information signals from each of the remote repeater stations 10 and selects the strongest audio signal from among all of those received. Usually, this is the signal having the best signal-to-noise ratio. Voter 18 is well known and typical examples thereof are shown in U.S. Pat. Nos. 3,729,781; 3,729,682; and 4,035,729. The subject matter of these three U.S. Patents is hereby incorporated as reference. A voter 18 that is in widespread use is the Voting Selector Panel LBI 30002B provided by the General Electric Company. Details thereof are set forth in its maintenance manual data file folder-DF 9025, available from the General Electric Company, Mobile Communications Business Division, Lynchburg, Va. The material contained therein is hereby incorporated by reference. A telephone line 23 couples the selected audio from voter 18 to central control console 22 where it is played through a speaker (not shown) so that the dispatcher can hear the voted signals detected by one of receivers 14. Indicator lights (not shown) indicate to the dispatcher which of receivers 14 is providing the strongest signal.
Console 22 is coupled via telephone lines 24 (24-1 and 24-2, respectively) to each of remote repeater stations 10. Lines 24-1 couples central control console 22 to remote repeater station 10-1 and line 24-2 couples central control console 22 to remote repeater station 10-2. The dispatcher can originate audio signals and couple them via lines 24 to either of remote repeater stations 10. This allows the dispatcher to engage in two-way communication with a mobile user via a dispatcher-selected remote repeater station 10.
Central control console 22 includes means for generating signals for controlling the various operating modes and functions of remote repeater station 10. Usually, these control signals are in the form of a sequence of tones. In the case of the MASTR R controller provided by the General Electric Company, a first high amplitude tone burst of 2175 Hz is used to secure access to a remote repeater station 10. A second lower amplitude tone burst, known as "function tone" turns on the remote repeater station 10 selected. A third tone or even lower amplitude than the function tone, maintains the remote repeater station 10 in a transmit mode while it is present. This transmit hold tone generally is 20 Db in amplitude less than the amplitude of the function tone. The transmit hold tone is present as long as the dispatcher's push-to-talk switch is depressed.
A control 26 (26-1 and 26-2, respectively for repeater station 10-1 and 10-2) interprets the tones generated at central control console 22 and coupled to the remote repeater station via line 24. After interpreting the tones, control 26 actuates, as appropriate, a relay 28 (28-1 and 28-2, respectively, for stations 10-1 and 10-2) through a relay control 30 (30-1 and 30-2, respectively, for stations 10-1 and 10-2). Control 26 also provides for receiver muting, and other mode controls as needed. Relay 28 couples the input of transmitter 12 to either the output of its associated receiver 14 or to audio line 24 from central control console 22. The control of switch 28 is responsive to the tone sequence from central control console 22 as interpreted by control 26.
The arrangement set forth in FIG. 1 assumes the use of standard remote repeater stations like those normally supplied in systems wherein each remote repeater station includes a transmitter and receiver. Remote repeater stations 10 must be equipped for decoding the signals generated by central control console 22 for enabling and disabling its repeat function.
If the dispatcher wants a transmitter 12 to repeat (re-transmit) the signal received by a receiver 14, he generates the appropriate sequence of tones which, when decoded, sets a memory element (typically, a flip-flop) in the remote repeater station. When set, this memory element enables circuitry within remote repeater station 10 which causes transmitter 12 to re-transmit signals received from its co-located receiver 14. In other words, control 26 is activated to couple the output of receiver 14 to the input of transmitter 12. To disable the repeat function, the dispatcher must issue a second tone sequence which resets the memory element and cancels the re-transmit mode of operation.
In order to properly operate the system shown in FIG. 1, the dispatcher observes indicator lights (not shown) on console 22 indicating which receiver 14 has been selected by voter 18 and manually enables the repeat function of the remote repeater station 10 associated with the selected receiver. Thus, there is an operational disadvantage in utilizing this arrangement. The dispatcher at central control console 22 must manually select the particular remote repeater station 10 that is to re-transmit a particular received transmission. It is necessary for a mobile user to contact the dispatcher and request repeater activation before he is able to communicate with another mobile user. The dispatcher must be careful to avoid simultaneously enabling more than one remote repeater station 10 at a time to prevent simultaneous transmission interference in areas of overlapping coverage. For an active system, the attention required to keep up with communications traffic can be quite burdensome to the dispatcher. In addition, the type of system set forth in FIG. 1 requires that a transmitter be co-located with each receiver. A transmitter can only re-transmit audio detected by its co-located receiver and not from any other receiver.
Generally, the type of system set forth in FIG. 1 does not allow a sufficient level of control from central control console 22. Since non-transmit functions do not have the positive control provided by the hold tone that is used in a transmit function tone sequence, the system is subject to falsing by a momentary power interruption which causes the memory element to change state. If a phone like 24 fails while the repeat function is enabled, the dispatcher has no way to turn off a remote repeater station 10 and has lost control of the system. A second known control arrangement for a plurality of remote stations is shown in FIG. 2.
Referring now to FIG. 2, there is shown an alternate control arrangement presently in use for a plurality of remote stations 30. Four remote stations 30-1, . . . 30-4 are shown. Stations 30-1 and 30-4 include only receivers 32 (32-1 and 32-4, respectively). However, remote stations 30-2 and 30-3 include both receivers 32 (32-2 and 32-3, respectively) and transmitters 34 (34-2 and 34-3, respectively). The output of each receiver 32 is coupled via a separate telephone line 36 (36-1, 36-2, 36-3 and 36-4, respectively) to a voter 38. As with voter 18 shown in FIG. 1, the strongest received signal is selected. This is usually the least noisy signal. The strongest received signal from voter 38 is coupled via telephone lines 40 and 42 to a central control console 44 and to a remote keying unit (RKU) 46, respectively. RKU 46 is suitably an LBI-4650 provided by the General Electric Company and fully described in its maintenance manual data file folder-DF 9025, the contents of which are incorporated herein by reference. Voter 38, in addition to selecting the strongest received information signal and coupling it to console 44 also generates a signal on one of a plurality of lines 39 coupled to a voter select input 50 of a transmitter selection unit (TSU) 52. Each of lines 39 correspond to one of receivers 32. A signal on a particular one of lines 39 therefore identifies the receiver providing the audio signal selected by voter 38. Voter 38 also provides a signal on a line 41 whenever it provides a signal on any of lines 30. In essence, the signal on line 41 is an "OR" function of the signals on lines 39.
RKU 46 includes a control tone generator 56 and a summer 58. Control tone generator 56 has an input coupled to a control input 48 of RKU 46 and an output coupled to one input of summer 58. A second input of summer 58 is coupled to line 42 for receiving the voted audio signal. Summer 58 adds the tones generated by control tone generator 56 to the voted audio signal on lines 40 and 42 and provides a composite signal on a line 54.
Control input 48 of RKU 46 is coupled to line 41 from voter 38. Thus, whenever voter 38 signals TSU 52 via one of lines 39, it provides a signal to control tone generator 56. Control tone generator 56 generates a sequence of tones similar to those that can be generated by central control console 44, which is similar in all relevant respects to central control console 22 described with reference to FIG. 1.
RKU 46 is therefore responsive to both the voter signal at its control input 48 and to the strongest received audio signal coupled thereto via line 42. The signal on output line 54 is a composite of both the strongest received audio signal and a tone sequence for controlling a remote station 30.
Central control console 44 includes means for generating a dispatcher originated tone sequence identical to that produced by control tone generator 56 within RKU 46, on a line 60 coupling it to a signal input of TSU 52. Line 54 from RKU 46 is coupled into line 60 so that the RKU signal will also be coupled to signal input 62 of TSU 52. Thus, TSU 52 can receive signals from either or both of RKU 46 and central control console 44. TSU 52 is suitably an LBI 4615 provided by the General Electric Company and described in its maintenance manual data file folder DF 9025, the contents of which are incorporated herein by reference. TSU 52 includes a relay switch 53 having a pole 55 coupled to its signal input 62. TSU 52 further includes a control 57 for actuating relay switch 53 so as to determine which of its throws will be coupled to pole 55. Control 57 is coupled to lines 39 from voter 38. The particular one of lines 39 having a signal thereon uniquely determines through the action of control 57 which throw of relay switch 52 will be coupled to pole 55. Each throw of relay switch 53 is coupled to a distinct line 64 (64-2 and 64-3, respectively coupled to remote stations 30-2 and 30-3). Thus, a signal on one of lines 39 determines to which of remote stations 30 signal input 62 of TSU 52 is coupled. An audio signal with control tones superimposed thereon appearing at signal input 62 is coupled to the appropriate remote station 30 for transmission. This audio signal can be either a received audio signal selected by voter 38 or a dispatcher-originated audio signal. A voted audio signal from a receiver 32 will have tones from control tone generater 56 superimposed thereon and dispatcher-originated audio will have control tones generated by central control console 44 superimposed thereon. Each remote station 30 having a transmitter 34 includes a control 66 for coupling a line 64 from TSU 52 to the input of its associated transmitter 34.
In this arrangement, the best quality audio signal selected by voter 38 is coupled to central control console 44 so that the dispatcher is able to hear a mobile user. For mobile-to-mobile communication, RKU 46 superimposes a transmit control tone sequence on the selected audio (strongest received information signal) and couples the combined signal to the TSU 52. RKU 46 generates a sequence of tones identical to that generated by console 44 when the dispatcher activates a push-to-talk key for activating a remote transmitter. TSU 52 uses the voted signal to route the strongest received information signal from voter 38 to the transmitter associated with the receiver selected thereby. The remote station 30 selected in accordance with the voted audio signal transmits the audio coupled to it via line 64. Thus, a remote station 30 associated with the receiver providing the strongest received audio signal, as selected by voter 38, re-transmits that audio signal. This arrangement utilizes duplex remote control stations and repeats selected audio with fully automatic transmitter selection. Capability to manually override the automatic selection is normally provided.
Utilizing an arrangement such as set forth in FIG. 2 frees the dispatcher for other duties. The system may be left in an automatic mode and each mobile will have access to repeaters as determined by voter 38. Since each of stations 30 is a remote control station, TSU 52 automatically prevents simulcast situations from developing. Since a transmit tone sequence is utilized for activating transmission by a transmitter 34, a station 30 unkeys when RKU 46 unkeys and a phone line failure of any of lines 40, 42, 54 and 60 cannot leave a remote station 30 in an enabled mode. The positive control provided by a hold tone of the transmit sequence of tone prevents a momentary interruption of power from causing a failure repeater enable situation.
The arrangement shown in FIG. 2 may be utilized when more than one receiver is associated with a single transmitter. This situation frequently is utilized when talk-out coverage is greater than talk-back coverage due to lower power of the mobile radios (such as personal radios). However, there are distinct disadvantages in utilizing the arrangements shown in FIG. 2.
The received information signal to be retransmitted must traverse phone lines from a receiver 32 to voter 38 and then traverse another phone line from TSU 52 to a transmitter 34. This use of tandem phone lines often requires special amplifiers to recover losses and special equalizers for maintaining acceptable frequency response and distortion levels. The extra equipment required for such tandem line use is not normally included in radio equipment and must be separately supplied at additional expense. Noise introduced into the communications links by the phone lines cannot be removed and is enhanced with tandem lines. With normal voice grade lines specified for point-to-point communication the degradation of the repeated signal is likely to be noticeable and may be severe. Higher grades of phone lines can be utilized but this requires additional and recurring expenses making such systems commercially impractical.